1. Field of the Invention
The invention relates to a method for producing a permanent mark in an optical element which consists essentially of a material that is transparent in the visible spectral region, and to an apparatus suitable for carrying out the method and to an optical element that can be produced with the aid of the method or the apparatus. A preferred field of application is the marking of spectacle lenses and other transparent optical elements for ophthalmics.
2. Description of the Related Prior Art
When being produced, spectacle lenses, which, irrespective of the material, are often also referred to as spectacle glasses are mostly provided with one or more permanent marks. A mark can include, for example, information relating to the manufacturer, a batch number or characteristics of optical elements. It is also possible to provide marks for identifying optical axes, centering points or other items of information defining the type of lens in order, for example, in the case of progressive lenses for the optician to be able to fit the spectacle lens acceptably into the spectacle frame provided therefor. Such marks are generally to be semivisible to the spectacle wearer or during normal use of the spectacles, and can have correspondingly small dimensions. Marks can also be located in a peripheral region of the spectacle lens which is severed during the fitting into the spectacle frame. Such marks can also be visible. Again, other materials that are transparent in the visible spectral region are sometimes provided with marks, for example lenses for multi-lens optics. In cases of interest here, a permanent mark is desired in order to ensure traceability.
Initially, marks were generated in a purely mechanical way by engraving (for example DE 35 34 920 C2). It is also known to generate marks in the form of etched marks by leeching out glass components after covering the areas which are not to be marked, for example with the aid of a lacquer. EP 0 103 217 describes the production of spectacle lenses with a reflection-reduction layer, a mark being generated in the reflection-reduction layer in the form of a cutout.
Soon after the commercial availability of lasers, a shift was made to using irradiation with high-energy laser radiation for the purpose of changing objects near the surface which are transparent in the visible spectral region such that permanent, visible marks are produced. This approach is set forth, for example, in U.S. Pat. No. 2,746,193 with reference to the example of the marking of glassware.
Given not excessively high power densities of the laser radiation and not excessively short pulses of the laser radiation, a near-surface removal of material which can lead to a visible, permanent mark is possible only when the material to be processed at least partially absorbs the laser wavelength used, that is to say is not completely transparent or completely free of absorption in the region of the operating wavelength of the laser radiation. Since the materials used for spectacle lenses scarcely absorb in the visible spectral region (typically between approximately 380 nm and 710 nm), and frequently also in adjacent spectral regions of ultraviolet radiation and/or infrared radiation, in the field of the marking of spectacle lenses specific use has been made of laser radiation whose operating wavelength lies clearly outside the visible spectral region.
Patent EP 0 307 874 B1 describes, for example, the use of an excimer laser with an operating wavelength of approximately 193 nm for the purpose of generating a mark in an anti-reflection layer system in such a way that the mark is visible only in reflected light. Since in the case of laser wavelengths of 193 nm or 248 nm such as are generated by excimer lasers ultraviolet light reacts aggressively with atmospheric oxygen to form ozone and thereby experiences an attenuation of the laser intensity, DE 10 2004 004 514 A1 proposes an apparatus for signing or marking spectacle lenses in the case of which the laser beam of an excimer laser is guided through a housing which is filled with a protective gas which does not react with the laser beam.
Japanese patent application with the publication number JP 2003 156667 AA describes the use of a frequency-multiplied solid state laser whose operating wavelength likewise lies in the ultraviolet spectral region because of the frequency multiplication.
Another class of laser apparatuses for marking spectacle lenses and comparable objects made from materials that are transparent in the visible spectral range makes use as light source of CO2 lasers which emit in the middle infrared region. By way of example, it is proposed in U.S. Pat. No. 4,219,721 to mark plastics lenses based on polymethyl methacrylate (PMMA) with the aid of infrared radiation from the wavelength region around 10.6 μm.
It is proposed in U.S. Pat. No. 5,767,483 to use CO2 lasers for the purpose of marking glass and other materials of comparable thermal conductivity.
A further known approach is to apply to the surface to be marked a thin layer which absorbs particularly strongly in a spectral region well covered by conventional lasers, and subsequently to irradiate it locally, as a result of which the absorption layer experiences a locally adhesive connection to the substrate. Such an indirect method is disclosed, for example, in U.S. Pat. No. 6,075,223.
It is likewise known to mark contact lenses by means of laser radiation (for example EP 946 896 B1). It is also known to generate marks in casting moulds for the production of contact lenses by means of laser radiation (JP 7186290A). In the last case, raised marks are produced on the contact lenses resulting from casting as negative moulds of the depressions generated by means of laser radiation.